The present invention concerns a device in connection with a riser for the transport of oil and/or gas, possibly including water, from a well head, transport pipe or similar structure on the sea bed to a platform.
Based on exploration activities in the North Sea in recent years, it seems today as if the era of large oil and gas finds is over. On the other hand, there have been many finds of a smaller size. At current oil/gas prices, they are not really commercially attractive/profitable, but in the future they may be interesting if oil prices increase and/or the technical solutions for extraction of oil become cheaper.
Many platforms which are already installed and in use have reserve risers for future developments. A common feature of these risers is that they were meant for large new fields. Their dimensions are therefore often much larger than what can be used for production from marginal fields with modest production and service life.
The situation is, therefore, that many risers which are held in reserve will never be used. In theory, it is conceivable for a transport pipe with a small diameter from a marginal production field to be connected directly to an existing reserve riser of a larger diameter. However, this is not possible as the authorities and the regulations require, among other things, that the riser and the production pipe can be reamed (pigged), which can be difficult or impossible to do at the transition from a large to a small pipe.
The alternative, therefore, is to lay a new riser which meets the statutory requirements for strength and installation. A new riser is, however, very expensive and may, as indicated above, contribute to reducing the incentive for developers to undertake the development of small oil and gas fields. In the worst case scenario, development may be stopped for lack of profitability.
The present invention represents a solution which makes it possible to use existing reserve risers with the pressure class and strength with which such risers were installed and which meet the requirements of the existing regulations. The solution makes it possible to perform pigging and is also less expensive to install and use compared with the installation of new risers.
In accordance with the present invention, the solution is characterised in that, in order to use a riser for the transport of oil from a small oil field with low oil/gas production where a transport pipe with a small diameter is used, a flexible pipe with generally the same internal diameter as the transport pipe is arranged inside the riser. At its upper and lower ends, the riser is provided with a coupling, which forms a seal between the transport pipe and the flexible pipe and also forms a seal at the ends of the riser so that it is closed.